With “where you really live” being a reference to Walt’s money, my prediction from last week was completely wrong. This is why I write about the science and not the plot. That said:

We know that Walt survives this encounter in the desert as we’ve seen him in the flash-forward. Does the fact that he has hair indicate that he’s beaten the cancer again, or that he’s stopped chemotherapy because there’s no point any more?

For what it’s worth, I think that Jesse will end up captured and cooking for Uncle Jack (and Operation Blue), and that Walt’s military-level arsenal is for storming the place and getting him out. So expect the exact opposite to happen next week.

Todd has inherited Walt’s equipment from his pest control days, but unfortunately not his skills. I couldn’t quite place the device used to check the meth purity at first:

It looks a little like a handheld microscope, but is missing the light source and probably wouldn’t work on such a thick sample.

It probably isn’t a spectrophotometer, as they would require an enclosed sample area.

Devices used to measure things like melting point and conductivity look quite different.

With the black plate for the sample, it looks a bit like a lab-on-a-chip or immunoassay device – but then where are the microfluidics and why the eyepiece?

The eyepiece is a real puzzle, since most digital measurement devices would display their output on a screen.

The refractive index of a substance is a number that describes how quickly radiation (light) travels through the substance. As you might expect, light tends to travel more slowly through denser substances (like water) than through less dense ones (like air). This is why a prism will break white light up into its constituent colours – refractive index index depends on wavelength. When the light enters the denser medium of the prism, the colours with shorter wavelengths (at the violet end of the rainbow) slow down more than the colours with longer wavelengths (at the red end) and separate out in a phenomenon called dispersion.

The refractive index of a solution depends on the concentration of whatever’s dissolved in it, so it can quite easily be used to confirm the purity of something. By comparing the refractive index of the “aquamarine” sample to that of a “blue” sample (or, more likely, to a table of known refractive indices for pure methamphetamine), Todd can make a fairly good guess as to the purity of his product.

However, Todd cannot be completely certain of his 76 % claim without knowing how the impurities in his product affect its refractive index. If he were measuring methamphetamine in a known solution (e.g. water) then he wouldn’t have a problem – it’s all the unknown stuff messing up the speed of light that’s going to send his errors skyrocketing.

Callback to Madrigal: Refractometry is also used to assess the sugar content (i.e. Brix number) of sugar-containing solutions.

Uncle Jack is little reckless in opening fire while Walt’s in the car – Hank and Steve are probably relatively safe behind the engine block (and, by the look of things, everyone is a terrible shot anyway), but the car doors protecting Walt may as well be made of tissue paper.